Back-illuminated image sensor with dishing depression surface

ABSTRACT

A fabricating method of a back-illuminated image sensor includes the following steps. First, a silicon wafer having a first surface and a second surface is provided, wherein a number of trench isolations are formed in the first surface, and at least one image sensing member is formed between the trench isolations. Then, a first chemical mechanical polishing (CMP) process is performed to the second surface using the trench isolations as a polishing stop layer to thin the silicon wafer. Because the polishing rate of the silicon material in the silicon wafer is different with that of the isolation material of the trench isolations in the first CMP process, at least one dishing depression is formed in the second surface of the silicon wafer. Finally, a microlens is formed above the dishing depression, and a surface of the microlens facing the dishing depression is a curved surface.

TECHNICAL FIELD

The present invention relates generally to image sensors, and moreparticularly relates to a back-illuminated image sensor and afabricating method thereof.

BACKGROUND

Generally, in a fabricating method for a back-illuminated image sensor,sensing elements, a signal processing member, a dielectric layer, andmetal wires are firstly finished on a front side of a silicon wafer.After that, a thinning process is performed on a back side of thesilicon wafer to thin a thickness of the silicon wafer from severalhundred micrometers to several micrometers, for example, from 750micrometers to 3 micrometers. Then, optical members such as color filterand microlens are formed on a back surface of the thinned silicon waferto introduce light beams from the back surface. In such a configuration,the light beams wouldn't be blocked by the metal wires. However, it isvery difficult to accurately control the thinning process such that thethinned wafer after thinned several hundred micrometers has goodcross-wafer uniformity and wafer-to-wafer uniformity. If the cross-waferis over-thinned or unevenly thinned, the sensors on the front surfacewould be damaged or reduce the performance of the optical members on theback surface of the silicon wafer, and also the wafer-to-wafer thicknessuniformity would affect the quality of the image sensors.

Therefore, how to solve the above problems and improve the yield rateand the performance of back-illuminated image sensors is an objective ofthe present invention.

SUMMARY

In one embodiment, a fabricating method of a back-illuminated imagesensor is provided. The method includes the following steps. First, asilicon wafer is provided and the silicon wafer includes a first surfaceand a second surface. A number of trench isolations are formed at thefirst surface, and at least one image sensing member is formed betweenthe trench isolations. Then, a first chemical mechanical polishing (CMP)process is performed on the second surface to thin the silicon waferusing the trench isolations as a polishing stop layer. In the first CMPprocess, the polishing rate of the silicon material in the silicon waferis different from that of the isolation material in the trenchisolations. As a result, at least one dishing depression is formed inportions of the second surface of the silicon wafer that are between thetrench isolations. Finally, a microlens is formed above the dishingdepression, and a surface of the microlens facing the dishingdepressions is a curved surface.

In another embodiment, a back-illuminated image sensor is provided. Theback-illuminated image sensor includes a silicon wafer and at least onemicrolens. The silicon wafer includes a first surface and a secondsurface, wherein at least one image sensing member and at least oneperipheral circuit are formed on the first surface. The microlens isdisposed above the second surface, and a surface of the microlens thatis adjacent to the second surface of the silicon wafer is a curvedsurface.

In the fabricating method of the back-illuminated image sensor, the CMPprocess, which is used to thin the silicon wafer has high polishing rateselectivity since the trench isolations are employed as the polishingstop layer. Thus, the thinned silicon wafer has good cross-waferthickness uniformity and wafer-to-wafer thickness uniformity. Inaddition, the surface of the microlens adjacent to the second surface ofthe silicon wafer of the back-illuminated image sensor is a curvedsurface. Therefore, the incident light can be accurately focused ontothe image sensing member thereby improving the performance of theback-illuminated sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more readily apparent to thoseordinarily skilled in the art after reviewing the following detaileddescription and accompanying drawings, in which:

FIG. 1A to FIG. 1E are schematic views illustrating a part of steps of afabricating method of a back-illuminated image sensor in accordance withan embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more specifically withreference to the following embodiments. It is to be noted that thefollowing descriptions of preferred embodiments of this invention arepresented herein for purpose of illustration and description only. It isnot intended to be exhaustive or to be limited to the precise formdisclosed.

FIG. 1A to FIG. 1E are schematic views illustrating a part of steps of afabricating method of a back-illuminated image sensor in accordance withan embodiment.

Referring to the cross sectional view shown in FIG. 1A, first, a siliconwafer 100 is provided. The silicon wafer includes a first surface 110and a second surface 120. A number of deep trench isolations 111 areformed in the first surface 110, and at least one image sensing member112 is formed between two deep trench isolations 111. At least oneperipheral circuit 113 is formed aside the image sensing member 112.After that, dielectric layers 114 a, 114 b and 114 c are sequentiallyformed on the first surface 110, and metal conducting wires M1, M2 andM3 are formed in the dielectric layers 114 a, 114 b and 114 c,respectively. Then, an electrode layer 115 is formed on the outmostdielectric layer 114 c, and an insulator layer 116 is also formed on theelectrode layer 116. In addition, a carrier wafer 199 is bonded onto theinsulator layer 116. In the present embodiment, a vertical depth d1 ofthe deep trench isolations 111 is not greater than 3 micrometers. Theimage sensing member 112 between the deep trench isolations 111, forexample, is a photodiode for receiving external light beams and convertsthe received light beams into electrical charges. The peripheral circuit1113, for example, is a signal process circuit consisting of metal oxidesemiconductor transistors for reading the electrical charges from thephotodiode and converting the electrical charges into digital imagesignals. It is to be noted that, the number of the dielectric layers andthe metal conducting wires on the first surface can be varied accordingto practical requirements when the present embodiment is implemented,and FIG. 1 is an exemplary illustration and is not intended to be usedto limit the method, type or number of the dielectric layers, the metalconducting wires and the peripheral circuit.

Referring to the cross sectional view shown in FIG. 1B, after thecarrier wafer 199 is bonded onto the silicon wafer 100; the siliconwafer 100 is inverted using the carrier wafer 199 as a support. Then, athinning process is performed on the second surface 120 to reduce athickness of the silicon wafer 100 to about 3 micrometers while doesn'texpose a surface of the deep trench isolations 111. The thinning processincludes a second CMP process, or an etching process, or the both. Forexample, the second CMP process can be performed firstly to reduce thethickness of the silicon wafer to about 40 micrometers, and then theetching process can be used to reduce the thickness of the silicon wafer100 to about 3 micrometers. Alternatively, the etching process can beperformed firstly and then the second CMP process is performed.Furthermore, the second CMP process and the etching process can also beperformed alternately to reduce the thickness of the silicon wafer. Thatis, the present embodiment doesn't limit the sequence of the thinningprocess. A primary objective of the thinning process is to quicklyreduce the thickness of the silicon wafer. Thus, it is not necessary toconsider the selectivity of the slurry used in the second CMP process orthe etchant used in the etching process, and the only thing should beconsidered is the polishing rate and the etching rate to the siliconmaterial. The process time of the thinning process is controlled. Forexample, the thinning process should be terminated when the thickness ofthe silicon wafer 100 is less than a predetermined value such as 3micrometers such that the deep trench isolations 111 are not exposed.

Referring to the cross sectional view shown in FIG. 1C, a first CMPprocess, which has high polishing rate selectivity, is performed on thesecond surface 120 to continuously thin the silicon wafer. The first CMPprocess, the deep trench isolations 111 is used as an polishing stoplayer. As the slurry used in the first CMP process has differentpolishing rate to the silicon material and the material of the deeptrench isolations 111 (for example, silicon dioxide), the deep trenchisolations 111 is not easily removed after the deep trench isolations111 are exposed. Thus, a variation of the reflecting light signalsand/or the electrical signals representing the polishing resistance canbe detected when the deep trench isolations 111 are exposed.Accordingly, the thinning process can be accurately controlled to avoidover-thinning or insufficient thinning. As a result, the thinned siliconwafer has both the good cross-wafer thickness uniformity and thewafer-to-wafer thickness uniformity. In detail, the silicon material 100of the silicon wafer 100 and the isolation material (e.g., silicondioxide) have different refractive indexes and different polishing ratesin the slurry used in the first CMP process. Therefore, in the presentembodiment, the highly selective first CMP process can be performedusing the deep trench isolations 111 as the polishing stop layer.

In addition, due to the high polishing rate selectivity, the first CMPprocess would produce dishing depressions 120 a in the second surface120 of the silicon wafer that is between the deep trench isolations 111.

Now referring to the cross sectional view shown in FIG. 1D, to preventthe recombination of the electron-hole pair produced by the imagesensing member 112 after receiving external light thereby avoidingreduce the amount of the effect electrical charges, an ion implantationcan be performed on the second surface 120. It is to be noted that thethinning process and the ion implantation may produce lattice defect(not shown) in the silicon wafer 100, and this lattice defect would leadto current leakage. To avoid the lattice defect, an annealing processcan be used to repair the lattice defect. After the annealing process,an anti-reflection layer 121 can be formed to reduce unwanted reflectionof incident light. A microlens 122 is formed above the dishingdepression 120 a. Because the microlens 122 can be formed to beconformal with the shape of the dishing depression 120 a, a surface ofthe microlens 122 facing the dishing depression 120 a is a curvedsurface. In other words, the microlens in the image sensor is a convexlens having light converging effect. According to the marching directionof incident light, surfaces of a convex lens can be divided into anincident surface and a light emitting surface. Convex lens has convexincident surface and flat light emitting surface is called convex planolens; convex lens has flat incident surface and convex light emittingsurface is called plano convex lens; and convex lens has convex incidentsurface and convex light emitting surface is called convex-convex lens.Among the above three types of convex lens, if the curvature radius isthe same and the incident light is in a parallel direction, theconvex-convex lens has the shortest focus length and the convex planolens has the longest focus length. In other words, if the same focuslength is required for the three types of convex lens, the curvatureradius of the convex plano lens should be the smallest and that of theconvex-convex lens should be biggest. That is, the curved surface of theconvex plano lens is more curved and the curved surface of theconvex-convex lens is relatively plane.

Compared with font-illuminated image sensor, the distance between themicrolens and the image sensing member of back-illuminated image sensoris shorter to improve the amount of incident light. As discussed above,the convex plano lens should has a relative small curvature radius toobtain shorter focus length. Thus, in the fabricating method of aback-illuminated image sensor, the curvature radius should be accuratelycontrolled to obtain precise focus length for efficiently converginglight onto the image sensing member. In the present embodiment, themicrolens 122 above the dishing depression 120 a may be plano convexlens or convex-convex lens relative to the marching direction of theincident light. The plano convex lens or convex-convex has a shorterfocus length than the convex plan lens when the curvature radius is thesame. Thus, in the fabricating process, it is easier to made the focuslength is between the microlens 122 and the image sensing member 112such that the incident light can be accurately converged onto the imagesensing member 112 thereby improving the photo-electric conversionefficiency and also the performance of the back-illuminated imagesensor. According to another aspect, in the highly polishing rateselective first CMP process, if the ratio of the polishing rate of thesilicon material to the polishing rate of the isolation material isdifferent, the produced dishing depression 120 a in the second surface120 and between the deep trench isolations 111 also has differentprofile. For example, the higher the polishing rate of the siliconmaterial is, the smaller curvature radius of the dishing depression isobtained; and the smaller curvature radius of the dishing depression isobtained; the smaller curvature radius of the microlens 122 facing thedishing depression 120 a is obtained. Therefore, the shape and thecurvature of the dishing depression 120 a can be controlled by adjustingthe composition of the slurry used in the highly polishing rateselective first CMP process. Accordingly, the light emitting surface ofthe microlens 122 that faces with the dishing depression 120 can beformed into different curvature radius. In the present embodiment, theratio of the polishing rate of the silicon material to the isolationmaterial in the highly selective first CMP process should be big enough,for example, bigger than 200.

It is to be noted that, the back-illuminated image sensor as shown inthe cross sectional view of FIG. 1D can also be fabricated by othermethods to form the curved surface of the microlens 122 that faces withthe second surface 120. For example, after the silicon wafer is thinned,the second surface is etched to form at least one groove. Then, ananti-reflection layer is formed on the second surface. In succession,another highly polishing rate selective CMP process is performed. Thepolishing rate of the anti-reflection layer is greater than that of thesilicon material. Thus, dishing depressions are formed in the surface ofthe anti-reflection layer in the groove. Finally, the microlens isformed above the dishing depression in the surface of theanti-reflection layer. As such, the surface of the microlens that faceswith the second surface is also a curved surface.

Referring to the cross sectional view shown in FIG. 1E, finally, thefabricating method of a back-illuminated image sensor may furtherincludes the following step: forming a first plane layer 123 on themicrolens 122. In addition, a metal shielding layer 124 is formed on thefirst plane layer 123 to prevent external light directly irradiating theperipheral circuit 113. The metal shielding layer 124 may consist ofaluminum, copper or titanium, and the present embodiment doesn't limitthe composition. After that, a color filter 125 is formed on the firstplane layer 123 and above the microlens 122. Finally, a second planelayer 126 and a second microlens 127 are optionally formed on the colorfilter 125 thereby obtaining a back-illuminated image sensor. It isworthy to note that, according to the marching direction of the externallight, the second microlens 127 is defined as a convex plano lens, andthe microlens 122 and the second microlens 127 constitute a two convexcombination. In the fabricating process, it is easier to adjust thecurvature radius of the light incident surface of the second microlens127 and accurately control the focus length. As a result, thephoto-electric conversion efficiency is improved and the performance ofthe back-illuminated image sensor is also improved.

In summary, according to the present embodiment, the thinning process ofthe silicon wafer can be accurately controlled thereby obtaining goodcross wafer thickness uniformity and wafer-to-wafer thicknessuniformity. Therefore, the focus length of the microlens can also beaccurately controlled, the photo-electric conversion efficiency isimproved. Thus, the back-illuminated image sensor provided in thepresent embodiment has better performance and quality.

While the invention has been described in terms of what is presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the invention needs not be limited to the disclosedembodiment. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

What is claimed is:
 1. A back-illuminated image sensor, comprising: asilicon wafer, comprising a first surface and a second surface, whereinat least one image sensing member and at least one peripheral circuitare formed in the first surface and at least one right-side up dishingdepression is formed in the second surface of the silicon wafer; and atleast one microlens, disposed above the right-side up dishing depressionformed in the second surface of the silicon wafer, and a surface of themicrolens facing the second surface of the silicon wafer is a smoothlycurving surface which is formed to be conformal with the right-side updishing depression.
 2. The back-illuminated image sensor of claim 1,wherein a plurality of trench isolations are formed in the first surfaceand the image sensing member is formed between the trench isolations. 3.The back-illuminated image sensor of claim 2, wherein the at least oneright-side up dishing depression is formed between the trenchisolations.
 4. The back-illuminated image sensor of claim 2, wherein adepth of the trench isolations in a direction perpendicular to the firstsurface is less than or equal to three micrometers.
 5. Theback-illuminated image sensor of claim 2, wherein an edge of the surfaceof the microlens is directly next to an end of one of the trenchisolations.
 6. The back-illuminated image sensor of claim 2, wherein anedge of the right-side up dishing depression touches an end of one ofthe trench isolations.
 7. The back-illuminated image sensor of claim 1,further comprising: an anti-reflection layer, disposed on the secondsurface; a first plane layer, disposed on the microlens and theanti-reflection layer; a metal shielding layer, disposed on the firstplane layer; and a color filter, disposed above the microlens, and onthe first plane layer.
 8. The back-illuminated image sensor of claim 7,further comprising: a second plane layer, disposed on the color filter;and a second microlens disposed on the color filter.